Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

An unusual 1,3-hydrogen shift from the metal center to the Cβ atom of the C3-chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ3-P,O,P-[xant(PiPr2)2]})2(μ-Cl)2][BF4]2 (1). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh2){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (2), which yields OsHCl(=C=C=CPh2){κ3-P,O,P-[xant(PiPr2)2]} (3) by selective abstraction of the Cβ–H hydrogen atom of the alkenylcarbyne ligand with KtBuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the Cβ atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=CIndPh){κ3-P,O,P-[xant(PiPr2)2]} (4) and the osmanaphthalene OsCl(C9H6Ph){κ3-P,O,P-[xant(PiPr2)2]} (5). Protonation of 4 with HBF4 leads to the elongated dihydrogen complex [OsCl(η2-H2)(=CIndPh){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (6), while the protonation of 5 regenerates 2. In contrast to 4, complex 6 evolves to a half-sandwich indenyl derivative, [Os(η5-IndPh)H{κ3-P,O,P-[xant(PiPr2)2]}][BF4]Cl (7). Phenylacetylene also provokes the 1,3-hydrogen shift in 3. However, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne to afford [Os(≡CCH=CPh2)(η2-HC≡CPh){κ3-P,O,P-[xant(PiPr2)2]}]Cl (8).


■ INTRODUCTION
Transition metal unsaturated carbene complexes, particularly vinylidene and allenylidene derivatives, are modern and powerful tools in organic and organometallic synthesis. Their use is allowing the development of previously inaccessible or difficult transformations, which simplifies the building of a diverse range of types of carbon−carbon and carbon− heteroatom bonds. 1 Other tools of paramount relevance are the transition metal hydride complexes. They are classical inorganic compounds, 2 which are increasingly used in homogeneous catalysis. 3 The reason for this fact is because they are ideal for setting unsaturated organic molecules at metal fragments, 4 can generate radicals with Markovnikov selectivity by H· transfer, 5 and have demonstrated a marked ability to functionalize C−H bonds as consequence of their capacity to activate σ-bonds. 6 Thus, complexes bearing both classes of ligands have an enormous potentiality, being the stabilization and control over their chemical properties a challenge of first magnitude.
The main problem for the development of the stoichiometric chemistry of these bifunctional compounds, which enables to understand the catalytic performance, is their low stability, since they are thermodynamically unstable with regard to the 1,2-insertion products (eq 1). 7 As a consequence, only a scarce number of hydride-vinylidene complexes of 8 group metals have been isolated and fully characterized so far, 8 mainly osmium derivatives, 8a−e,g−j whereas the known hydrideallenylidene compounds are reduced to the cations [OsH( CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + and [OsH(CC CPh 2 )(η 2 -HCCH) (P i Pr 3 ) 2 ] +9 and the neutral iridium(III) complexes IrHCl(CCCPhR) (P i Pr 3 ) 2 (R = Ph, t Bu), 10 although only some reactivity of the first of them has been investigated. 11 Transition metal allenylidene complexes can be grouped into electrophiles and nucleophiles, according to the reactivity of the unsaturated C 3 -chain. While electrophiles have attracted great attention, nucleophiles have been scarcely studied. The nucleophilic allenylidenes are characterized by addition of electrophiles to C β . With alcohols, the great majority of them are inert. 1b However, the allenylidene ligand of cation [OsH(CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + displays supernucleophilic behavior, which allows the reduction of the C α − C β double bond of the unsaturated chain (Scheme 1). The 1,addition of the O−H alcohol bond to the metal center and the C β atom affords an alkoxide-hydride-carbyne intermediate, which leads to dihydride-carbyne species by β-hydrogen elimination of the alcoholate. The subsequent migration of one of the hydrides to the carbyne C α atom gives the reduction product hydride-alkenylcarbene. 11a,d The hydride ligand of [OsH(CCCPh 2 )-(CH 3 CN) 2 (P i Pr 3 ) 2 ] + is certainly efficient for fixing unsaturated organic molecules beside the allenylidene ligand. It reacts with terminal alkynes to afford alkenyl-osmium(II)-allenylidene derivatives, which evolve into metalacyclopentapyrrole compounds in acetonitrile (Scheme 2). Their formation implies the genesis of three carbon−carbon bonds. The C α and C γ atoms of the cumulene are coupled with the C α and C β atoms of the alkenyl group, whereas the C β atom of the C 3 -chain is attacked by the electrophilic C(sp) atom of the solvent. 9 The mentioned reactions of [OsH(CCCPh 2 )-(CH 3 CN) 2 (P i Pr 3 ) 2 ] + evoked us a question: what is the driving force of this unusual behavior, the charge of the complex, the weak coordinating ability of the acetonitrile ligand, or both? During some years, we unsuccessfully looked for a metal fragment that would allow us to address this question. In 2010, we reported the preparation of the ether-diphosphine 9,9dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant-(P i Pr 2 ) 2 ), which can keep the trans-P−Os−P arrangement observed in the cation, by means of its coordination κ 3 -P,O,Pmer. 12 In addition, Weller's group has demonstrated in the past years that POP-diphosphines are flexible hemilabile ligands. 13 According to such ability, xant (P i Pr 2 ) 2 is adapted to the requirements of the reactions in which its complexes participate, enabling the necessary geometrical transformations on the metal coordination sphere through changes in its coordination mode. 14 As proof-of-concept validation, besides species bearing the diphosphine κ 3 -P,O,P-mer coordinated, 15 complexes with the ligand in fashions κ 3 -P,O,P-fac, 16 κ 2 -P,Pcis, 17 and κ 2 -P,P-trans 14b,18 have been also isolated. This is allowing to perform reactions 19 and to isolate compounds 20 initially forbidden, and as result, interesting catalysts for a wide range of processes are being discovered. 6b,c,19,21 The proved flexibility and great coordinative versatility of xant (P i Pr 2 ) 2 inspired us to use it to address the preparation of a neutral hydride-osmium(II)-allenylidene complex. Its stabilization would permit to study its behavior toward alcohols, water, and terminal alkynes and to in this way answer the question above.
This paper reports the preparation of a neutral hydrideosmium(II)-allenylidene complex structurally related to the cation [OsH(CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + and, in order to address the question raised, analyzes its behavior toward alcohols, water, aldehydes, and phenylacetylene, which promote an unusual 1,3-hydrogen shift from the metal center to the C β atom of the cumulene.

■ RESULTS AND DISCUSSION
Preparation of the Neutral Hydride-Osmium(II)-Allenylidene Complex. Scheme 3 summarizes the strategy employed to obtain the target compound. We selected the tetrahydride dimer [(Os(H···H){κ 3 -P,O,P-[xant (P i (P i Pr 2 ) 2 ]}]BF 4 . The fac-disposition of the diphosphine in 1 stabilizes the dimeric structure with regard to the mer-coordination due to a decrease of the steric hindrance experienced by the isopropyl substituents of the unsaturated fragments. However, the coordination mer is favored over fac for mononuclear saturated metal centers. Acetonitrile breaks the chloride bridges of the dimer, saturating the osmium center. At a time, the compressed dihydrides are approached to form an elongated dihydrogen, whereas the ether-diphosphine changes its disposition from fac to mer, as a consequence of the disappearance of the steric hindrance. 16 In this context, it should be mentioned that unsaturated osmium-dihydride complexes, which afford dihydrogen species by coordination of electron poor Lewis bases, react with propargyl alcohols to give hydride-osmium-alkenylcarbyne derivatives. The π-CC coordination of the alkynol at the vacancy promotes its tautomerization to hydroxyvinylidene, which undergoes dehydration and addition of the acidic atom of the generated dihydrogen. 11d According to this, 1,1-diphenyl-2-propyn-1-ol reacts with the tetrahydride dimer, in fluorobenzene at 80°C, to form the expected hydride-osmium-alkenylcarbyne [OsHCl-(CCHCPh 2 ){κ 3 -P,O,P-[xant (P i Pr 2 ) 2 ]}]BF 4 (2), through intermediates A and B. Complex 2 was isolated as a red solid in 76% yield. Despite the expected acidity of the hydride ligand of 2, the treatment of its tetrahydrofuran solutions with 1.1 equiv of K t BuO produces the selective abstraction of the C β −H hydrogen atom of the alkenylcarbyne ligand. The deprotona-tion affords the desired allenylidene ligand. Complex OsHCl-(CCCPh 2 ){κ 3 -P,O,P-[xant (P i Pr 2 ) 2 ]} (3) was isolated as a green solid in 86% yield. The deprotonation is reversible; the addition of 1.0 equiv of HBF 4 to dichloromethane solutions of 3 quantitatively regenerates 2.
Complex 2 was characterized by X-ray diffraction analysis.  (2) Å, which is fully consistent with an Os−C(1) triple bond formulation. 22 The alkenylcarbyne proposal is supported by the bond lengths and angles within the carbon donor ligand. Carbons C(1) and C(2) are separated by 1.423(3) Å, whereas the C(2)−C(3) distance is 1.360(3) Å. The angles around C(2) and C(3) lie in the range 112−127°. In agreement with the presence of a hydride ligand, the 1 H NMR spectrum, in dichloromethane-d 2 , at room temperature shows a triplet ( 2 J H−P = 16.4 Hz) at −5.59 ppm. In the low field region of the spectrum, the most noticeable signal is a singlet at 5.55 ppm corresponding to the C(sp 2 )−H hydrogen atom of the alkenyl group. In the 13 C{ 1 H} spectrum the Os−C(sp) resonance appears at 271.5 ppm, as a triplet with a C−P coupling constant of 5.6 Hz, whereas the alkenylcarbyne C(sp 2 ) resonances are observed at 166.9 and 130.8 ppm as singlets. The 31 P{ 1 H} NMR spectrum contains a singlet at 52.9 ppm, as expected for equivalent P i Pr 2 groups.
Complex 3 has been also characterized by X-ray diffraction analysis. Figure 2 shows a view of the molecule. The coordination around the osmium atom resembles that of 2, with the allenylidene ligand in the position of the alkenylcarbyne group; i.e., a distorted octahedral arrangement with P(1)−Os−P(2), C(1)−Os−O(1), and H(01)−Os− Cl(1) angles of 160.88(6)°, 177.8(2)°, and 164(2)°, respectively. The diphenylallenylidene ligand is bonded to the metal center in a nearly linear fashion (Os−C (1) (2), and C(2)−C(3) distances of 1.858(6), 1.261(8), and 1.351(8) Å, respectively, compare well with those reported for the previously structurally characterized osmium-allenylidene complexes. 9,21b,23 In agreement with them, C(1)−C(2) and C(2)−C(3) are about 0.05 Å shorter and longer, respectively, than the bond length expected for a carbon−carbon double bond (about 1.30 Å), which suggests a notable contribution of the canonical form [M] − CC C + Ph 2 to the structure of the C 3 -chain. In accordance with the presence of hydride and allenylidene ligands, the IR spectrum of the molecule contains the corresponding characteristic ν(Os−H) and ν(CCC) bands at 2090 and 1863 cm −1 . In the 1 H NMR spectrum, in dichloromethane-d 2 , at room temperature, the hydride resonance appears as a triplet ( 2 J H−P = 17.4 Hz) at −8.82 ppm. In the 13 C{ 1 H} NMR spectrum, the C 3 -chain gives rise to three triplets at 154.8, 242.5, and 256.1 ppm, with C−P coupling constants of 2.4, 10.4, and 4.1 Hz, which were assigned to the C γ , C α , and C β atoms, respectively. The 31 P{ 1 H} spectrum displays a singlet at 27.6 ppm.
Complex 4 is a notable example of stable hydrideindenylidene, which does not evolve to the half-sandwich indenyl species. Figure 3 shows its structure which proves the cyclization of the cumulene of 3 and the mutual disposition cis of the hydride ligand and the C(sp 2 ) atom of the carbocycle. The coordination polyhedron around the osmium atom is the expected octahedron for a six-coordinate d 6 24 The presence of the hydride ligand in the molecule is also supported by the 1 H NMR spectrum, in dichloromethane-d 2 , at 223 K, which contains a triplet ( 2 J H−P = 21.9 Hz) at −18.54 ppm. In the 13 C{ 1 H} NMR spectrum, the resonance corresponding to C(1) appears at 232.4 ppm. The 31 P{ 1 H} NMR spectrum displays a singlet at 53.3 ppm for the equivalent P i Pr 2 groups.
Complex 5 is also certainly noticeable, since is a new member of the scarcely represented family of metalanaphthalene derivatives within the class of metalaaromatic compounds. 25 Figure 4a shows its structure, which proves the formation of the osmacycle. The coordination polyhedron around the osmium atom can be rationalized as a distorted octahedron with the ether-diphosphine κ 3 -P,O,P-mer coordinated (P(1)−Os−P(2) = 156.79 (13) (1) = 177.5(4)°). The bond lengths in the bicycle reveal that from the three resonance forms contributing to its structure, a−c (Figure 4b), the form a is the most significant followed by b.   context, it should be noted that such disposition is observed in the four complexes, 2−5, which could be related to the greater π-donor ability of oxygen with regard to chlorine and the πacceptor capacity of the C-donor ligands. The existence of a markedly dominant resonance form can explain the low NICS(0) and NICS (1) values computed, 2.4 and −2.6 ppm, respectively, which are however in agreement with those found in other metalaaromatic complexes of this class. 26 The 13 C{ 1 H} NMR spectrum of the green crystals, in dichloromethane-d 2 at room temperature, also supports the dominant contribution of the resonance form a to the structure of the metalabicycle. In agreement with an almost double character of the Os−C(1) bond, the resonance corresponding to C(1) appears at 248.1 ppm, while the signal due to C(5) is observed at higher field, 168.9 ppm, as expected for an Os−C(sp 2 ) almost single bond. The 31 P{ 1 H} NMR spectrum shows a singlet at 33.4 ppm, in accordance with the equivalence of the P i Pr 2 groups disposed mutually trans. Isomerization reactions from 3 into 4 and 5 are water-, alcohol-, and aldehyde-catalyzed competitive processes. The molar ratio between the isomeric products is independent of the catalyst. In order to understand this fact, we carried out the isomerization in the presence of D 2 O and methanol-d 4 . In both cases, we obtained the 7:3 mixture of the monodeuterated isomers 4-d 1 and 5-d 1 with the deuterium atom bonded to the C(2) atom of the compounds (Scheme 5); i.e., the C β atom of the cumulene of 3.
The position of the deuterium atom, analogous in each compound, points out that the first step is common for both isomerization reactions and involves a catalyst-mediated 1,3hydrogen shift from the metal center to the C β atom of the cumulene of 3. To gain insight about this unusual migration and the subsequent cyclization processes, we carried out DFT calculations at the dispersion-corrected SMD(fluorobenzene)-B3LYP-D3//SDD(f)/6-31-G** level (Figures S35−S37; see computational details in the Supporting Information). The changes in free energy (ΔG) were calculated at 298.15 K and 1 atm. Figure 5 shows the computed energy profile, whereas Scheme 6 gathers all the intermediates involved in the reaction.
The direct migration of the hydride to the C β atom of the allenylidene ligand, through a four-center transition state, is energetically prohibited since it must be overcome a barrier of 71.4 kcal·mol −1 ( Figure S35). The migration in two consecutive 1,2-hydrogen shifts, via an allenyl intermediate ( Figure S36) is also energetically forbidden. Although the activation energy for the formation of the allenyl species is reduced to 28.1 kcal·mol −1 , the transition state for the second migration lies 51.5 kcal·mol −1 over 3. However, the proton shuttle formed by two water molecules consecutively associated by means of hydrogen bonds, significantly reduces  Figure S38), much less tensioned than that for the direct migration, which affords the fivecoordinate osmium(0) intermediate C.
In spite of its saturated character, the latter oxidatively adds the ortho-CH bond of a phenyl substituent of the alkenylcarbyne ligand in one step through the transition state TS C-D , which lies 23.3 kcal·mol −1 over 3. The approach of the C−H bond to the metal center causes the dissociation of the oxygen atom of the etherdiphosphine, before of the C−H cleavage. The oxidative addition generates the osmanaphthalyne D, which is 7.3 kcal mol −1 less stable than 3. The existence of this class of compounds has been experimentally demonstrated by Jia, Lin, and co-workers. 27 Osmanaphthalyne D bears both fragments of the C−H bond activation disposed cis to the Os−C triple bond of the metalacycle. In agreement with the Jia and Lin calculations, the 1,2-carbon-migration leads to the hydrideosmium-indenylidene derivative 4, whereas the 1,2-hydrogenmigration gives the osmanaphthalene 5. The barriers are similar, 26.0 kcal mol −1 for the former and 26.9 kcal mol −1 for the second. As expected for the composition of the isomerization mixture, complex 4 is slightly more stable than 5, 2.8 kcal mol −1 . In spite of this small difference, they do not interconvert after isolation, as corresponds to the very high barrier for the isomerization on both sides. The intramolecular insertion reactions initially afford the respective five-coordinate species E and F, which subsequently coordinate the oxygen atom of the ether-diphosphine to yield the isomerization products. The different behavior of 3 and the cation [OsH(CC CPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + toward alcohols is evident. On the basis of experimental observations and DFT results, this fact can be rationalized on both the difference in coordination ability between an acetonitrile ligand and the oxygen atom of the diphosphine and the difference in charge between the complexes. The greater coordination capacity of the ether group of the diphosphine with regard to the acetonitrile ligand prevents the coordination of the alcoholate, resulting from the protonation of the C β atom of the cumulene. In this context, it should be mentioned that deuterium labeling experiments and theoretical calculations on the hydrogenation of the cation indicate that the β-hydrogen elimination in the coordinated alkoxide group is the key for the reduction, because the formation of a dihydride-carbyne species, with a cis disposition of the carbyne to both hydrides (trans between them), is essential to the 1,2-hydrogen shift from the metal center to the C α atom of the carbyne (Scheme 1). 11a In addition, the neutral character of 3 with respect to the cationic nature of [OsH( CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + increments the basicity of the metal center in the former, which increases the activation energy for the hydride migration from the metal center to the C-donor ligand 28 and, at a time, favors the oxidative addition of the phenyl C−H bond in C.
Protonation of 4 and 5. Indenylidene complexes display a marked tendency to evolve to indenyl derivatives by 1,2-shift of an 1-e − donor ligand, including chloride, from the metal center to the carbenic carbon atom. 27,29 In addition, it has been argued that one of the difficulties in the synthesis of metalanaphthalene compounds could be due to its lower stability relative to the indenyl derivatives. So, the stability of 4 and 5 first surprised us and then encouraged us to promote their transformation to indenyl species, in particular that of 4. In this context, we noted that the carbenic carbon atom of alkylidene-osmium(II) complexes has amphiphilic character, reacting with both nucleophiles and electrophiles, including H + . 30 Thus, we decided to study the protonation of both 4 and 5.
Addition of 1.0 equiv of HBF 4 to a dichloromethane solution of 4 at 223 K immediate and quantitatively affords the   4 ]Cl (7 in Scheme 7a). In accordance with a dihydrogen-to-indenylidene migration of H + , the partially deuterated species 6-d 1 affords a 1:1 mixture of the 7-d 1 isomers shown in Scheme 7b. The double salt was isolated as orange crystals and characterized by X-ray diffraction analysis. Figure 6 gives a view of the cation. The distribution of ligands around the osmium atom can be described as a four-legged piano stool geometry. The indenyl ligand, which is coordinated by the fivemembered ring, occupies the three-membered face while the ether-diphosphine and the hydride ligand lie in the fourmembered face. The P i Pr 2 groups are disposed in transoid position (P(1)−Os−P(2) = 107.32(7)°), and as a consequence, the oxygen atom and the hydride ligand must be situated in the other two vertices of the face (O(1)−Os− H(01) = 150(3)°). The P(1)−Os−P(2) angle compares well with the angles observed in other compounds bearing a κ 3 -P,O,P-fac coordinated ether-diphosphine, 14b,16 even with those displaying a κ 2 -P,P-cis mode, 17 and the P−Os−P angle usually found in osmium(IV) complexes with four-legged piano stool geometry and a transoid disposition of two phosphine ligands. 32 However, it significantly deviates from the ideal angle for a P−Os−P cis disposition in an octahedral osmium(II) derivative. This difference could explain why the hydride complex 4 does not evolve to an indenyl-osmium(II) derivative, while the elongated dihydrogen compound 6 affords the double salt of the indenyl-osmium(IV) cation 7. The influence of polydentate ligands bite angles on the coordination polyhedron of the complexes, and therefore on the stability of the different oxidation states of the central ion, is well demonstrated. 33 The 1 H and 31 P{ 1 H} NMR spectra, in dichloromethane-d 2 , at room temperature are consistent with the structure shown in Figure 6. Thus, the 1 H contains at a doublet of doublets ( 2 J H−P = 36.1 and 32.4 Hz) at −12.57 ppm, due to the hydride ligand, whereas the 31 P{ 1 H} NMR displays two doublets ( 2 J P−P = 11.9 Hz) at −12.6 and −23.0 ppm, corresponding to the inequivalent P i Pr 2 groups.
The osmanaphthalene complex 5 also reacts with HBF 4 . However, in contrast to 4, the protonation regenerates the hydride-alkenylcarbyne 2 as a consequence of the attack of the proton to the bridgehead C(5) atom of the bicycle and an 1,2hydrogen shift from C(1) to the metal center. According to Organometallics pubs.acs.org/Organometallics Article this, the addition of DBF 4 to the dichloromethane-d 2 solution of 5 selectively leads to 2-d 1 containing a deuterium atom at one of the ortho-carbon atoms of a phenyl substituent of the alkenylcarbyne ligand (Scheme 8).
Reaction of 3 with Phenylacetylene. Treatment of toluene solutions of hydride-osmium(II)-allenylidene complex with 4 equiv of the alkyne, for 2 days, at room temperature, leads to the π-alkyne-osmium(0)-alkenylcarbyne [Os( CCHCPh 2 )(η 2 -HCCPh){κ 3 -P,O,P-[xant (P i Pr 2 ) 2 ]}]Cl (8). The salt was isolated as a brown solid in 58% yield. The reaction implies a 1,3-hydrogen shift from the metal to the C β atom of the allenylidene ligand, which produces the Os 2+ -to-Os 0 reduction of the central ion, the displacement of the chloride anion by the alkyne, and a change in the coordination of the ether-diphosphine from κ 3 -P,O,P-mer to κ 3 -P,O,P-fac (Scheme 9).  = 16.9 Hz, in agreement with inequivalent P i Pr 2 groups.
The formation of 8 appears to be consistent with the isomerization of 3 to 4 and 5, at least on an initial examination. Moreover, one could think that 8 is the result of trapping the intermediate C of Scheme 6 by means of the coordination of phenylacetylene. However, it should be noted that, although the C(sp)−H hydrogen atom of the alkyne is also fairly acidic, phenylacetylene has not an equivalent to the oxygen of the catalysts promoting the isomerization (water, alcohols, and aldehydes) to interact with the hydride ligand of 3 and to approach the acidic proton to the C β atom of the cumulene. In view of this inconsistency, we decided to carry out the reaction of 3 with PhCCD. Under the same conditions as that employed to form 8, 8-d 1 was quantitative and selectively obtained (Scheme 10).
The position of the deuterium atom at the coordinated alkyne of 8-d 1 indicates that the 1,3-hydrogen shift in this case occurs by a different manner to those previously discussed. A feasible alternative could involve the reductive elimination of HCl as consequence of the acidification of the metal center, due to the initial replacement of the oxygen atom of the etherdiphosphine by the alkyne. Once the reduction has taken place, the recoordination of the ether linker, now κ 3 -P,O,P-fac with the oxygen atom trans to the cumulene, and the subsequent

■ CONCLUDING REMARKS
This study has revealed the existence of a 1,3-hydrogen shift in the elusive hydride-metal-allenylidene complexes, which is responsible for the isomerization of the cumulene to indenylidene 34 and the transformation of the hydrideallenylidene unit into the metalaaromatic bicycle metalanaphthalene. The hydrogen shift, which has an activation energy too high to occur in a concerted manner, is catalyzed by water, alcohols, and aldehydes. Phenylacetylene also provokes the 1,3-hydrogen shift; however, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne, preventing its evolution into indenylidene or metalanaphthalene. This study has also illustrated a new behavior of transition metal allenylidene complexes toward alcohols. Until now, these compounds had shown three different conducts. Those with electrophilic nature form α,β-unsaturated alkoxycarbene derivatives, as a result of the 1,2-addition of the O−H bond of the alcohols to the C α −C β double bond of the allenylidene, nucleophilic allenylidenes are inert, 1a and the cation [OsH( CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + undergoes reduction of the C α −C β double bond as a consequence of a hydrogen transfer reaction from the alcohol to the complex. 11a The transformation of 3 into 4 and 5 represents an alternative conduct, alcohol-induced isomerization.
It was thought so far that the reactivity of an allenylidene ligand was only a consequence of its nucleophilicity or electrophilicity, which is imposed by the coligands of the complex. Thus, allenylidene ligands of similar electronic nature should display analogous behavior. Complex 3 and the cation [OsH(CCCPh 2 )(CH 3 CN) 2 (P i Pr 3 ) 2 ] + bear allenyli-dene ligands, which have a common characteristic: the strong nucleophilic character of the central carbon atom of the C 3chain. Nevertheless, they show different behavior due to different abilities of the coligands. The poor coordinating capacity of the acetonitrile ligand of the cation allows the reduction of the C α −C β double bond, while the association of the hydride ligand of 3 with the oxygen atom of the alcohol permits to lower the activation barrier for the 1,3-hydrogen shift from the metal to the C β atom of the cumulene; i.e., the coligands of allenylidene complexes are not innocent; they can have a direct participation in the reactions of the C 3 -chain.
In summary, a new reactivity pattern for hydrideallenylidene complexes has been observed, which is of interest in connection with the isomerization phenylallenylidene-toindenylidene and the transformation hydride, indenylidene-toindenyl. Furthermore, it can help to systematize the preparation of metalanaphthalene derivatives.
■ EXPERIMENTAL SECTION General Information. All reactions were carried out with exclusion of air using Schlenk-tube techniques or in a drybox. Instrumental methods and X-ray details are given in the Supporting Information. In the NMR spectra (Figures S1−S34) the chemical shifts (in ppm) are referenced to residual solvent peaks ( 1 H, 13 (2). Complex 1 (500 mg, 0.33 mmol) in fluorobenzene (4 mL) was treated with 1,1-diphenyl-2-propyn-1-ol (550 mg, 2.64 mmol), in the presence of 4 Å molecular sieves (2 g). After 2h, at 80°C , the resulting suspension was separated from the molecular sieves by decantation with a canule. Then, the liquid phase was removed and the dark red solid was washed with diethyl ether (3 × 2 mL) and pentane (6 × 3 mL) and dried under vacuum. Yield: 475 mg (76%). Crystals suitable for X-ray diffraction analysis were obtained by slow cooling of a fluorobenzene solution from 80°C to room temperature. Preparation of OsHCl(CCCPh 2 ){κ 3 -P,O,P-[xant-(P i Pr 2 ) 2 ]} (3). Complex 2 (500 mg, 0.53 mmol) and K t BuO (65 mg, 0.58 mmol) were dissolved in precooled THF (10 mL, −30°C). The green solution was stirred for 1 h at this temperature. Then, it was warmed to room temperature and the solvent was removed under a vacuum. The residue was treated with dichloromethane (5 mL